Japanese bank Mizuho has admitted losing the confidential data of 270,000 account holders.
Mizuho Financial Group, which owns the retail bank, said it had lost customer account numbers and names at 167 branches over several years, according to the Financial Times on Wednesday.
The bank is said to have suffered problems integrating systems and managing data since it was formed three years ago.
Mizuho has 30 million customers, more than any other Japanese bank. The company inherited its customer data when it was formed through the merger of Fuji Bank, Dai-Ichi Kangyo Bank and the Industrial Bank of Japan in 2002.
The bank, which is conducting an internal investigation, has said that the data was probably not stolen.